Adjusting Boat Speedometers

Adjusting Boat Speedometers

Accurate boat speed is very important in skiing. 
How can I tell if my speedometer is right?
How can I adjust my boat speedometer
Is there a way to adjust the boat speedometer without a slalom course?
Can I use a GPS to adkust my speedometer?
The normal way to tell if your boat speedometer is accurate involved timed runs through a slalom course.

A slalom course is a known length. 849.7 feet. The time it should take at various speeds is accurately known. Using a stopwatch, you can drive through a slalom course at a constant speed, and time how long it takes from the entrance gates to the exit gates. You can then compare the actual time you measure with the time it should take according to the speed reading on your speedometer.

Some boat speedometers are adjustable. Airguide is the defacto standard in speedometers and these have a front adjustment knob that allows you to calibrate then by trial and error during timed runs through the slalom course.

The following table indicates how long the timed runs should take at various speeds:

mph km/h    1st       1st         2nd        2nd         full
           segment   range      segment     range        course
24  40      9.81    9.57-10.06   13.50    13.17-13.85    23.31
26  43      9.13    8.92-9.34    12.56    12.27-12.86    21.68
28  46      8.53    8.35-8.72    11.74    11.49-12.00    20.27
30  49      8.01    7.89-8.13    11.02    10.85-11.19    19.03
32  52      7.55    7.44-7.66    10.38    10.24-10.54    17.93
34  55      7.13    7.05-7.22     9.82     9.70-9.94     16.95
36  58      6.77    6.69-6.84     9.31     9.21-9.42     16.08

The table lists segment times and tolerances. The full course time is the one normally used for adjusting speedometers. The mph numbers are approximate. For example, 34 should be 34.2 when converted from kilometers per hour. Full course timing is from entrance gates to exit gates. Segment timing is from entrance gates to ball 3 and from ball 3 to exit gates.

It is a good idea to adjust the speedometer at your normal skiing speed. SInce speedometers are not perfectly linear, adjusting them at your normal ski speed results in the most accurate reading at that speed.

For example, someone that skis at 36mph normally would do a timed run through the course at a constant speed with their boat speedometer reading 36mph. If the measured time throught the course was more than 16.08 seconds, then their boat speedometer must be indicating a speed faster than the boat is actually traveling. You would adjust it to read slightly slower using the front adjusting knob on an Airguide speedometer, and repeat the timed run through the course.

What if I don't have acess to a slalom course?

There are a few ways to solve this.

You can use a handheld GPS unit. While GPS is not 100% accurate at measuring distances, a 12 channel GPS unit is pretty good at displaying current speed. Find a long stretch of open water and drive at a constant speed at about normal ski speed. The handheld GPS should read a steady speed and you can adjust your speedo to match the GPS.

You could make a measured distance either with markers in the water or somewhere near shore where you can time between marks that are a known distance apart. You will have to calculate the times it should take for that distance and adjust your speedometer accordingly.

You could run alongside a boat with known accurate speedometers. Have the other driver travel at a fixed speed, usually your normal ski speed. When you are both traveling the exact same speed, adjust your speedometers to read the same.

As a last ditch effort, you can get somewhere close with your tachometer. An inboard tournament boat travels approximately 1mph for each 100 rpm. So at 3600 RPM you should read approximately 36 mph. This approach is VERY approximate. Actual boat speed varies depending on the propeller, the load in the boat, the transmission gear ratio etc. This approach serves only as a good starting point.

Terry Jones,